Definition: To be delighted
Example: Maria was tickled pink to have gotten front row tickets to see Bruce Springsteen
The tickling described in this idiom has nothing to do with somebody’s fingers on your belly. This has everything to do with the other meaning of tickling, that of being entertained or gratified. This usage of the word dates back to the 17th century.
However, the combination of tickling with the colour pink is much more recent, dating back only to the first years of the 20th century. And why pink? Why not blue, or orange, or in Iddy’s case, green? It simply is the colour of flushed cheeks when somebody is particularly happy or excited.
Apparently, there is a more extreme version of this delight. Tickled to death is another, somewhat rarer phrase in use. It means the same thing though the stakes sound much higher!